The connection of a variable power source to a power grid can lead to the introduction of unwanted power changes resulting from the variability in generated power. Such variability in generated power may arise due to the intermittent nature of renewable energy resources, such as wind, tidal and solar farms. These power changes may lead to grid instability and thereby affect the quality of the transmitted power in the power grid, which has economic implications for the power supplier and the end user.
According to a first aspect of the invention, there is provided a power transmission network, for interconnecting at least one variable power source and at least one AC or DC electrical network, comprising: at least one DC transmission link for DC power transmission between at least one network side converter and at least one source side converter; at least one AC transmission link for AC power transmission from the or each respective variable power source to at least one source side converter; at least one source side converter including: a DC connecting point operably connected to the or each respective DC transmission link; and an AC connecting point operably connected to the or each respective AC transmission link; at least one network side converter including: an AC or DC connecting point for connection to the or each respective AC or DC electrical network; and a DC connecting point operably connected to the or each respective DC transmission link; and a control system, wherein at least one network side converter is designated as a first converter, and the control system is configured to operate the or each first converter in a DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point with respect to the power generated by the or each respective variable power source.
A variable power source may be any power source, such as an intermittent energy source, that is capable of generating a variable power. Examples of an intermittent energy source include, but are not limited to, a wind farm, a tidal farm and a solar farm.
The or each AC electrical network may be or may include an AC power grid or system. The or each DC electrical network may be or may include a DC power grid or system.
During operation of the power transmission network, a variation in power generated by one or more connected variable power sources may arise. This may be due to, for example, the intermittent nature of renewable energy resources such as wind, tidal and solar farms.
For economic reasons, instead of controlling the amount of power generated by the or each variable power source, the power transmission network is configured to accommodate the variation in power generated by the or each variable power source, thereby transmitting all of the power generated by the or each variable power source to the or each AC electrical network.
For example, at least one source side converter may be designated as a second converter, and the control system may be configured to operate the or each second converter in an AC voltage control mode as an AC slack bus to control a magnitude and/or frequency of an AC voltage of the or each respective AC transmission link at a steady-state value and thereby facilitate variation of a power transfer between its AC and DC connecting points to accommodate a variation in power generated by the or each respective variable power source. Operation of the or each second converter in the AC voltage control mode facilitates generation of a stable AC voltage waveform in the or each respective AC transmission link and thereby allows the or each variable power source to synchronize to the or each respective stable AC voltage waveform. Such synchronization ensures that any power generated by the or each variable power source will be accommodated by the or each source side converter and thereby injected into the power transmission network for transmission to the or each AC electrical network.
Operation of the or each second converter in the AC voltage control mode however means that the DC voltage at the DC connecting point of the or each second converter is uncontrolled if all of the degrees of freedom available to the or each second converter is used to operate the or each second converter in the AC voltage control mode. Such circumstances require control of the DC voltage at the DC connecting point of the or each respective network side converter in order to enable operation of the power transmission network.
The inclusion of the control system in the power transmission network according to embodiments of the invention enables control of the DC voltage at the DC connecting point of at least one network side converter by designating at least one network side converter as a first converter and operating the or each first converter in the DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point with respect to the power generated by the variable power source. This permits optimization of power transmission in the DC transmission link, such as reducing transmission losses in the DC transmission.
It will be understood that variation of the DC voltage at the DC connecting point may be a direct result of control of the DC voltage at the DC connecting point or be an indirect result of control of another DC voltage at another point in the power transmission network.
The transmission of power from the or each variable power source to the or each AC electrical network requires a voltage slope across the or each respective DC transmission link to cause transfer of power from the or each source side converter to the or each respective network side converter. In other words, to transmit power from the or each variable power source to the or each AC electrical network, the DC voltage at the DC connecting point of the or each source side converter must be higher than the or each respective network side converter. A high power transfer from the or each source side converter to the or each respective network side converter requires a high voltage slope across the or each respective DC transmission link, while a low power transfer from the or each source side converter to the or each respective network side converter requires a low voltage slope across the or each respective DC transmission link.
Transmission losses during transmission of power via the or each DC transmission link may be reduced by increasing an average DC voltage of the or each DC transmission link to be near or at its maximum allowable level, which may be dictated by the voltage rating at the DC connecting point of each converter operably connected to the DC transmission link or the voltage rating of any other equipment connected to the DC transmission link. A reduction in transmission losses is achieved by operating the or each respective first converter in the DC voltage control mode to increase the DC voltage at its DC connecting point and thereby increase the average DC voltage of the or each DC transmission link to be near or at its maximum allowable level when the DC voltage at the DC connecting point of at least one second converter is uncontrolled.
However, the variability in power generated by the or each variable power source, together with the requirement for a voltage slope across the or each DC transmission link, may result in an increase in DC voltage at the DC connecting point of at least one converter above a safe level (e.g. a voltage rating at the DC connecting point) that results in unsafe operation of the power transmission network.
As mentioned above, the inclusion of the control system in the power transmission network according to embodiments of the invention enables operation of the or each first converter in the DC voltage control mode to vary the DC voltage at its DC connecting point with respect to the power generated by the variable power source. Such operation of the or each first converter in the DC voltage control mode not only enables control of the DC voltage at the DC connecting point of each converter to stay below or at a safe level across the range of variation of power generated by the or each variable power source, but also enables the increase of the average DC voltage of the or each respective DC transmission link during a decrease in power generated by the or each variable power source to reduce a DC current flowing in the or each respective DC transmission link so as to reduce transmission losses for a given power generated by the or each variable power source, thus optimizing power transmission in the power transmission network. It follows that, in order to keep transmission losses at a minimum value, the DC voltage at the DC connecting point may be continuously varied to be kept at a maximum value for a given power generated by the or each respective variable power source.
Hence, the ability of the or each first converter to operate in the DC voltage control mode allows optimization of the operation of the power transmission network according to embodiments of the invention, and therefore results in a more efficient, reliable and cost-effective power transmission network.
In contrast, omission of the control system from the power transmission network according to embodiments of the invention removes the ability of the or each first converter to operate in the DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point with respect to the power generated by the or each respective variable power source. Consequently, in order to ensure safe operation of the power transmission network and maintain the required voltage slope across the or each DC transmission link, the DC voltage at the DC connecting point of the or each network side converter must be at all times fixed at a value that corresponds to the maximum level of power generated by the or each variable power source. This however results in sub-optimal operation of the power transmission network because, whenever the power generated by the or each variable power source falls below its maximum level, the power transmission network is incapable of increasing the fixed DC voltage at the DC connecting point of the or each network side converter to reduce transmission losses.
The control system may be configured to operate the or each first converter to maintain a DC voltage of the or each respective DC transmission link at a DC voltage limit when varying the DC voltage at its DC connecting point. The DC voltage limit may be defined by a voltage rating of the DC transmission link or by a value that is lower than the voltage rating of the DC transmission link by a predefined voltage safety margin.
In embodiments of the invention the control system may be configured to operate the or each first converter to maintain a DC voltage at the DC connecting point of the or each source side converter at a DC voltage limit when the or each first converter is operated in the DC voltage control mode. The DC voltage limit may be defined by a voltage rating of the DC connecting point of the second converter or by a value that is lower than the voltage rating of the DC connecting point of the second converter by a predefined voltage safety margin.
Whilst the predefined voltage safety margin limits the extent to which the transmission losses in the power transmission network can be reduced through use of embodiments of the invention, configuring the control system in this manner not only further enhances the reliability of the power transmission network, but allows an operator to readily ensure the operation of the or each first converter in the DC voltage control mode complies with specific safety parameters through modification of the predefined voltage safety margin.
Operation of the or each first converter in the DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point with respect to the power generated by the or each respective variable power source may be carried out in different ways. For example, the control system may be configured to receive: at least one power or direct current measurement of the power transmission network and to operate the or each first converter in the DC voltage control mode in accordance with the or each power or direct current measurement; and/or a predicted or dispatched power generation value from the or each variable power source and to operate the or each first converter in the DC voltage control mode in accordance with the predicted or dispatched power generation value or with a value that varies from the predicted or dispatched power generation value by a predefined power safety margin.
It will be appreciated that the or each power measurement can be derived from other measurements. For example, the or each power measurement can be a product of voltage and current measurements.
The or each power or direct current measurement of the power transmission network may be measured at any point of the power transmission network, such as a point of connection to the or each variable power source or the AC or DC connecting point of a source or network side converter.
The use of the power safety margin in the operation of the or each first converter in the DC voltage control mode minimizes any adverse effect of any error in the predicted or dispatched power generation value might have on the DC voltage at the DC connecting point of the or each first converter.
In further embodiments of the invention the control system may be configured to operate the or each first converter to maintain the DC voltage at its respective DC connecting point to be continuously lower than the DC voltage at the DC connecting point of the or each respective source side converter when power is being transmitted from the or each variable power source to the or each AC electrical network. Configuration of the control system in this manner ensures complete evacuation of the power generated by the or each variable power source into the power transmission network.
The control system may be configured to operate the or each first converter to vary an AC voltage magnitude or a reactive power at its AC connecting point when the or each first converter is operated in the DC voltage control mode.
The availability of an additional degree of freedom to the or each first converter permits its operation to vary an AC voltage magnitude or a reactive power at its AC connecting point while being operated in the DC voltage control mode, thus adding to the functionality of the or each first converter and thereby further improving the efficiency of the power transmission network.
In still further embodiments of the invention the control system may be configured to receive at least one DC voltage measurement of the power transmission network and to operate the or each first converter in a DC voltage correction mode to vary a DC voltage at its DC connecting point so that the or each DC voltage measurement of the power transmission network match a predefined DC voltage profile. This allows the or each first converter to be operated to vary the DC voltage at its DC connecting point in the event that the or each DC voltage measurement does not match the predefined DC voltage profile.
The or each DC voltage measurement of the power transmission network may be measured at any DC point of the power transmission network, such as any point along a DC transmission link or a DC connecting point of a source or network side converter.
The configuration of the control system may vary depending on specific requirements of the power transmission network. For example, the control system may include a global controller for controlling a plurality of converters, at least one local controller for controlling at least one converter, or a combination thereof. The global controller may be located remotely from each converter and may be configured to communicate with each converter via telecommunications links. The or each local controller may be located in the vicinity of at least one converter. The global controller may be configured to communicate with at least one local controller via telecommunications links.
The power transmission network may be configured in various ways depending on its purpose.
The power transmission network according to embodiments of the invention may be configured for interconnecting a variable power source and an AC electrical network. More particularly, in embodiments of the invention, the power transmission network may include: a DC transmission link for DC power transmission between a network side converter and a source side converter; an AC transmission link for AC power transmission from the variable power source to a source side converter; a source side converter including: a DC connecting point operably connected to the DC transmission link; and an AC connecting point operably connected to the AC transmission link; and a network side converter including: an AC connecting point for connection to the AC electrical network; and a DC connecting point operably connected to the DC transmission link.
In this manner the power transmission network according to embodiments of the invention is configured as a point-to-point power transmission network.
In such embodiments of the invention the source side converter may be designated as a second converter, and the control system may be configured to operate the second converter in an AC voltage control mode as an AC slack bus to control a magnitude and frequency of an AC voltage of the AC transmission link at a steady-state value and thereby facilitate variation of a power transfer between its AC and DC connecting points to accommodate a variation in power generated by the variable power source.
The power transmission network according to embodiments of the invention may be configured for interconnecting at least one variable power source and at least one AC electrical network. More particularly, in other embodiments of the invention, the power transmission network may include: a DC transmission link for DC power transmission between at least one network side converter and at least one source side converter; first and second DC terminals, the DC transmission link being configured to operably interconnect the first and second DC terminals; at least one AC transmission link for AC power transmission from the or each respective variable power source to a or each respective source side converter; at least one source side converter including: a DC connecting point operably connected to the first DC terminal; and an AC connecting point operably connected to the or each respective AC transmission link; and at least one network side converter including: an AC connecting point for connection to the or each respective AC electrical network; and a DC connecting point operably connected to the second DC terminal.
In this manner the power transmission network according to embodiments of the invention is configured as a multi-terminal power transmission network.
In such embodiments of the invention the or each source side converter may be designated as a second converter, and the control system may be configured to operate the or each second converter in an AC voltage control mode as an AC slack bus to control a magnitude and frequency of an AC voltage of the or each respective AC transmission link at a steady-state value and thereby facilitate variation of a power transfer between its AC and DC connecting points to accommodate a variation in power generated by the or each respective variable power source.
In embodiments of the invention including a plurality of network side converters, at least one network side converter not designated as a first converter may be designated as a third converter, and the control system may be configured to operate the or each third converter in a DC power control mode to control a DC power at its DC connecting point. This permits the operation of the power transmission network to maximize use of the plurality of network side converters when at least one of the network side converters is not required to be designated as a first converter, thus improving the efficiency of the power transmission network.
In further embodiments of the invention including a plurality of network side converters, at least two network side converters may be each designated as a first converter. When at least two network side converters are each designated as a first converter, the control system may be configured to operate each first converter in the DC voltage control mode in accordance with a voltage-current or voltage-power droop characteristic and/or to coordinate the operation of the first converters in the DC voltage control mode. Such operation of each first converter improves the stability of the operation of the power transmission network.
In addition to being configured to transmit power from at least one DC transmission link to at least one AC electrical network, the power transmission network according to embodiments of the invention may be configured to transmit power from at least one AC electrical network to at least one DC transmission link. In still further embodiments of the invention including a plurality of network side converters, the control system may be configured to simultaneously operate: a subset of the plurality of network side converters to transfer power from the or each respective DC transmission link to the or each respective AC electrical network; and a subset of the plurality of network side converters to transfer power from the or each respective AC electrical network to the or each respective DC transmission link.
In use, the control system may be rendered incapable of controlling the or each respective network side converter as a result of, for example, a loss of communication between the control system and the or each respective network side converter or a breakdown in the control system.
At least one network side converter may be configured to generate a fixed DC voltage at its DC connecting point in response to the control system being rendered incapable of controlling the or each respective network side converter. At least one network side converter may be configured to receive at least one electrical measurement of the power transmission network and operate in a DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point and thereby facilitate variation of a power transfer between its AC and DC connecting points to accommodate the variation in power generated by the or each respective variable power source, in response to the control system being rendered incapable of controlling the or the respective network side converter.
Configuring at least one network side converter in this manner ensures safe operation of the power transmission network according to embodiments of the invention in the event of the control system being rendered incapable of controlling the or each respective network side converter.
A first power transmission network according to an embodiment of the invention is shown in FIG. 1.